Acetonitrile is a by-product formed during the production of acrylonitrile. The principal process for the production of acrylonitrile is by ammoxidation of propylene in the vapor phase over an oxidation catalyst. The reactor effluent typically contains anywhere from one to eight percent acetonitrile. Acrylonitrile and acetonitrile may also be produced by the reaction of hydrocyanic acid with acetylene or the reaction of acrolein with ammonia.
Acrylonitrile and acetonitrile are separated from the reactor effluent by various distillation procedures. For example, U.S. Pat. No. 3,399,120 and U.S. Pat. No. 3,264,197 discloses a method for the purification of acrylonitrile and acetonitrile by water extractive distillation.
Another byproduct formed during the above process is cyanides. These cyanides are typically found as hydrogen cyanide, but can additionally be in other forms such as cyanohydrins. When acrylonitrile and acetonitrile are separated, a percentage of the cyanides appear in the crude acetonitrile stream. These must be removed, or as known in the art, "killed" down to the ppm level for specification acetonitrile.
The prior art method of removing these cyanides has been by the addition of caustic and ferrous sulfate as described in U.S. Pat. No. 3,201,451. Prior to this addition, however, the crude acetonitrile stream resulting from the acrylonitrile operation must be azeotropicly distilled to remove a substantial portion of the water contained therein.
Ferrous sulfate and caustic react with the cyanides to form ammonium ferrocyanide. After the acetonitrile has been removed, the waste stream containing water and ammonium ferrocyanide has proven to be a difficult stream to dispose of environmentally. While free cyanides will degrade in waste treatment such as the use of bioponds, this complex cyanide will not.
The present invention's use of aldehydes, specifically formaldehyde, in place of ferrous sulfate not only removes the cyanides from the crude acetonitrile stream, but also provides a waste stream that is relatively easy to dispose of.